Spang-Chalfant pipe connections used during the 1950's, had a pin nose surface formed axially arcuate to seal on a conical box seat, which resulted in “line-contact” between them, so it soon failed by corrosion if not earlier by minute mechanical damage on that “line seal”. My “Graloc patents” U.S. Pat. Nos. 2,766,998 and 2,766,999 taught resilient pressure aided pin-nose surface-seals and they stopped the all too frequent oil well blowouts that were occurring in the Gulf of Mexico. Another application of those patents was for a threaded pipe connection licensed to J&L Steel Co. which used a Grayloc Seal Ring between conical seats in the box and pin of tubular connections. In 1960 I designed similar Grayloc Connections to seal the thirty thousand pipe connections of the NPR Nuclear Reactor at Hanford Wash., the reactor that produced the plutonium that helped win the Cold War, after which, it was retired after serving its 30 year design life. Numerous pipe connections today use a pin nose to seal on a box seat, but they are delicate and extremely susceptible to damage and corrosion because they are so thin, the pin nose formed on lesser diameters than the least pin thread root diameter. Additionally, pin nose seals whose ends butt against the box seat cannot seal over the full tension and compression stress range of the pipe body. A wide assortment of pipe connections having pin nose seals may be seen in the World Oil Magazine Tubular Tables published each year, all being formed on diameters smaller than the least thread root diameter, such that no swaged connection had a rating above 75% of pipe strength before filing of my co-pending patent application PCT/US27/001154 which depicted swage configurations in FIGS. 5 and 6 and in claims 11, 24-27 that provide high strength swage boxes that seal on threads, whereas, my invention provides in addition, a high strength swaged pipe connection having a pin-nose seal that reduces the seal to a diameter less than the thread diameter.
Swaged box connections used over the years have typically been rated at 65% and a few were nervously rated around 75% when formed on thick walled pipe. They could not claim higher ratings because thread engagement did not extend completely through the box wall, but stopped in a swaged portion formed on the same taper as the threads, which reduced the critical area at the neck of the box to no more than 75% on common wall thicknesses, if the machined threads were perfectly positioned both axially and radially with regard to the swage wall. However, such perfect positioning cannot be maintained during production threading, so actual efficiencies of prior art swaged connections have been less than 75%. Pin run-out threads were introduced about 1950 on API 5B Buttress Pin's that mate with fully formed box threads in couplings having outer diameters larger than the pipe, to provide a pin strength equal to the pipe strength, but to my best knowledge and belief, run-out threads have not been used at the least diameter of the box thread to maximize box strength of a swage connection, before application date of my PCT application noted above.
It is common practice in the manufacture of pipe threads to bevel or wipe the first thread of the box and pin so as to eliminate the sharp, narrow width starting thread which is highly subject to handling damage and the resulting galling that would occur, if the thread is not wiped.